1. Field of the Invention
The present invention relates to a microchannel plate and a photomultiplier tube, such as an image intensifier.
2. Description of the Related Art
An image intensifier is for intensifying an extremely weak optical image several ten thousands of times to enable observation of the optical image. The image intensifier is used for two-dimensional measurement of extremely weak light, such as a nightvision.
This apparatus is produced under the assumption that it will be used under conditions with extremely weak light. Under stronger light, problems such as halo and flare develop. Halo is a phenomenon wherein a bright circular ring-shaped area appears around a strong spot of light. Flare is a phenomenon wherein dark areas around the strong light spot appear bright.
Now, halo will be described in greater detail with reference to FIGS. 1(a) and 1(b). When a bright light spot 41 enters a photocathode 40 of the image intensifier, an intensified light spot 61 is produced on a fluorescent screen 60. A circular area, or halo 62, around the light spot 61 also appears bright on the fluorescent screen. The halo 62 includes four concentric halo components 63, 64, 65, and 66 with differing luminance. FIG. 2 shows an example of luminous distribution. When the light spot 61 has a diameter of about 0.15 mm, the circular halo 62 will appear with a diameter of about 1.0 mm. When the luminance of the spot 61 is about 200 nit, the luminance of the halo 62 will be 2 nit or less. Thus, the luminance of the halo 62 is 1/100 or less the luminance of the spot 61.
A weak light spot 61 will result in only a weak halo 62 so that no problems arise. However, a relatively strong light spot 61 will produce a strong halo. Dark places around the spot, where no light is incident, conspicuously brighten, thereby lowering the picture quality. This is a characteristic of image intensifiers, which needs improvement.
Details of the halo are described in the paper "MIL-I-49052D 3.6.9, 4.6.9."
Japanese Patent Publication Kokoku No. 63-29781 describes a method for electrically suppressing halo. According to this method, a current of electrons entering the fluorescent screen is detected. Voltages, applied to a microchannel plate, are feed-back controlled so that the electron current does not exceed a certain value. This can suppress generation of surplus electrons on the microchannel plate and therefore can suppress the halo.
Japanese Patent Publication Kokai No. 2-33840 has analyzed the halo phenomenon as described below. In image intensifiers, photoelectrons of light spots photoelectrically converted in the photocathode are accelerated and multiplied in a microchannel plate. The multiplied electrons are then accelerated in an acceleration electric field developed between the microchannel plate and the fluorescent screen. The electrons then strike the fluorescent screen, which then emits fluorescence. At this time, some electrons scatter off an aluminum metal backing on the fluorescent screen and reflect back toward the microchannel plate. The reflected electrons reenter the acceleration electric field which pushes them into the fluorescent screen. The fluorescent screen emits fluorescence as a result. Thus reflected and then reentered electrons generate the halo light.
Based on the above-described analysis of the halo generation, document No.2-33840 has proposed one method for suppressing the reflected electrons. According to this method, light element such as carbon is deposited on the metal backing on the fluorescent screen.